Heat 1 teaspoon oil in a Dutch oven over medium-high heat. Add sausage and cook, turning often, until lightly browned, about 4 minutes. Transfer to a cutting board and slice into 1/2-inch pieces. (The sausage will not be thoroughly cooked, but will continue cooking later.) dd the remaining 2 teaspoons oil to the pan and heat over medium heat. Add cabbage, sliced fennel, onion, garlic powder, fennel seed and pepper and cook, stirring often, until the cabbage has wilted slightly, about 3 minutes. Add potatoes and cook, stirring occasionally, until the potatoes are heated through, 2 to 4 minutes. dd broth, vinegar and mustard and stir until the mustard is incorporated; bring to a simmer. Place the sausage on top of the cabbage mixture, cover, reduce heat to medium-low and cook until the sausage is cooked through and the vegetables are tender, 7 to 10 minutes. Stir in chopped fennel fronds and serve.